A real-time clock is a device that keeps track of time and can communicate the time and date to other devices electronically. When power is removed from a system, the real-time clock derives its power from an alternate source (such as a battery) so that it can maintain its sense of time. The alternate power source will eventually weaken to the point that it cannot adequately supply power to the real-time clock. The real-time clock will then lose its sense of absolute time. When the real-time clock and system regain power the real-time clock must evaluate its concept of time to understand if it is valid or not. It will report this status to the system.
An analog circuit that detects a loss of power is the most common method for invalidating the contents of a real-time clock. The analog circuit monitors the voltage applied to the real-time clock circuits and senses when the voltage drops below a set threshold at which point the contents are marked as invalid. Such an analog circuit cannot exist in a pure digital circuit.